1. Field of the Invention
The present invention relates to a power supply controller controlling the status of connection between a load and a battery pack formed by combining a plurality of secondary batteries, and, in particular, to a technology for controlling the status of connection between a battery pack and an inverter in an electric vehicle traveling by converting DC power from a battery pack, i.e. a driving power source installed on electric vehicles such as, in particular, pure electric vehicles (PEV), hybrid electric vehicles (HEV), and hybrid electric vehicles with fuel cells and secondary batteries, etc., to AC power using the inverter and supplying it to a motor.
2. Description of Related Art
Overdischarging and overcharging has to be avoided when using secondary batteries, such as nickel hydrogen secondary batteries, lithium secondary batteries, etc. If a secondary battery is seriously overdischarged or overcharged, the electrolysis of the electrolyte inside the battery generates gas, which reduces the useful life of the secondary battery and degrades the secondary battery.
On the other hand, the method used for battery control in pure electric vehicles (PEV), where propulsive power is generated by a motor based on energy from a battery pack formed by combining a plurality of secondary batteries, as well as in the so-called hybrid electric vehicles (HEV), in which both an engine and a motor are installed, consists not only in using power running control (discharge control) for obtaining propulsive power, but also in using regenerative control (charge control) for recovering braking energy to the motor the vehicle uses for travel.
For example, Japanese Patent No. 3360499 describes a regenerative brake control device installed in an electric vehicle including a motor. The regenerative brake control device includes means for determining a charging power upper limit value based on the secondary battery condition, means for controlling the upper limit of a control target for regenerative braking force based on the charging power upper limit value and the rotational speed of the motor, and means for reducing the control target for regenerative braking force when the voltage of the battery exceeds a predetermined permissible value.
In addition, JP 2003-219510A describes a charge/discharge control device including charge/discharge power limiting means for controlling charge/discharge power so as to avoid exceeding a charge/discharge power upper limit value, which changes depending on the temperature and amount of charge stored in the secondary batteries.
Furthermore, JP H10-94101A describes an electric vehicle including emergency forced shutoff means for automatically performing a forced shutoff of an electric storage device supplying electric power to a motor when the vehicle encounters a predetermined emergency.
Based on the above, it is believed that it is essential to carry out control so as to maintain a permissible charge/discharge power value, and a permissible charge/discharge voltage value for a battery in accordance with the condition of the secondary battery, i.e. its voltage, temperature, and stored charge. In addition, it is believed that it is essential to shut off electric power to the motor to ensure the safety of the vehicle and, at the same time, in some cases, urge the user to repair it under circumstances causing overcharging, overdischarging, malfunctioning of the electric power control devices of the vehicle or severe battery life degradation.
Incidentally, due to the significant electric power requirements of secondary batteries used for driving electric vehicles, it is common to use battery packs made up of a plurality of serially connected single cells. As described in JP H11-248757A, using a flying capacitor circuit for a plurality of voltage sources allows for voltage measurement while maintaining insulation between the measurement system and the voltage sources with the help of a simple circuit.
In addition, according to the conventional technology, the electric power or voltage of a battery pack are monitored and, when any of those exceed a permissible charging power value or a permissible upper limit voltage value, or when they drop below a permissible discharging power value or a permissible lower limit voltage value, the connection between the battery pack and the motor is shut off.
However, in the above-described conventional technology, the battery pack was divided into blocks and the voltage of the battery pack was calculated by adding up voltages measured by a voltage sensor for each block. In addition, the determination of whether or not, for instance, the electric power of a battery pack exceeded a permissible charging power value was carried out based on an integrated value by computing the product of the electric current measured by a current sensor and the battery pack voltage obtained by such summation. Consequently, the problem with the above-described conventional technology was that computation and decision-making required time and detection was impossible when electric power exceeded a permissible charging power value within a short time period in a pulsed fashion. This resulted in a reduction in the useful life of the secondary batteries and caused their degradation.